Most stone is made over millions of years, cooked in the core of our planet. That stone then erodes over time due to wind, acids found in rain and groundwater, and other natural weatherings. Chalk is a type of limestone formed by the shells of microscopic marine organisms.
The process of erosion can produce beautiful features in earth’s crust, like arches, stacks, and caves. This process usually takes tens of thousands of years.
But we can make a limestone cave in just a matter of minutes. All you need for this DIY chalk cave is a block of chalk and vinegar.
The vinegar, a weak acid, reacts with the calcium compound, dissolving it. It then forms carbon dioxide gas, water, and an aqueous calcium solution. This same process takes place for real caves, but the acid in rain and ground water is much more diluted, therefore taking much much longer to see the results.
As far as we can tell, the ocean has been slightly basic for hundreds of millions of years. However, that seems to be changing, and the culprit may surprise you. Everyone has heard of carbon dioxide. It’s the thing we breathe out, what plants crave, and there’s this thing called the greenhouse effect which you’ve probably heard of by now.
CO2 concentrations have certainly changed in the past based on our examinations of ice core samples. However, they are certainly on the rise now. Credit: NASA, UCR, RUSD
What we do know for sure, is that carbon dioxide is on the rise. While the most often talked about consequence of this is a trend of increasing global temperature, there is another important concern, and it has to do with our water. There are many ways that we can measure water including temperature, pollutant content, clarity, and salinity, but this particular issue focuses on acidity.
Water is made up of two parts hydrogen and one part oxygen, leading the the chemical formula H2O. In pure water, these molecules occasionally break apart due to the following reaction:
On the left side, we’ve got our old pal water. In the middle, the double sided arrow means that this reaction can go both ways, depending on the ratio of water to its parts. Pure water is neutral, meaning that this reaction is the only source of H+ and OH– ions1 in the water. If anything throws this balance out of whack, the water will become basic if more OH– ions are added, or acidic if more H+ ions2 are added.
So what does all of this have to do with carbon dioxide (CO2)? When the CO2 gets in contact with water–which covers most of the Earth–it combines with the H2O by the following reaction.
This forms Carbonic acid, which then donates its extra hydrogen to the water like this:
In the previous paragraph, we talked about how anything that can add H+ to the water will cause it to become acidic. The resulting molecule above is known as carbonic acid because it has these extra hydrogen ions to donate.
While this all seems pretty abstract, you can watch it happed below. This water is poured over dry ice or solid carbon dioxide. Solid carbon dioxide turns directly into a gas at room temperature. The water also has an indicator solution that will cause it to turn from neutral green to red if it is acidic, and blue to deep purple if it is basic.
We measure this using the PH scale, which runs from zero to 14 with low numbers indicating higher acidity. It is also a logarithmic scale, which means that changing from a seven to a six on the scale corresponds to an increase of ten times to the acidity! For the past 300 million years, the ocean has been at a steady–and slightly basic–PH of 8.2. Over the past 200 years, it has dropped to about 8.1, which doesn’t seem like much on this scale, but corresponds to an increase of about 30% to the number of acidic ions in water. Even without further increase to atmospheric CO2, this trend should accelerate to a PH of 7.8 by the year 2100, an increase in acidic ions of 126%!
What does all of this mean for us living on Earth? No one is quite sure. Most organisms can only tolerate a narrow range of acidity, and there are many species that live in the ocean. Simulating these conditions in a lab is pretty easy, so we can identify some at risk species right away. Coral reefs, in particular, are known to be very sensitive to acidification.
Dissolved carbon dioxide can interfere with the calcification process that allows shells and exoskeletons to grow. Over the course of 45 days, this sea butterfly slowly dissolves in a lab simulation of acidic ocean water. Courtesy of David Littschwager/National Geographic Society. Similar research continues today.
Ions are just atoms or groups of atoms with a net charge, meaning they have an extra electron if negative, or lack an electron if positive. Ions are much easier to dissolve in water than neutral molecules.
Note: The freed hydrogen ions will actually combine with surrounding water molecules, forming Hydronium (H3OH+) which is usually denoted H+.
Puns abound in our Valentine’s day video, but we’re not about to let a holiday get in the way of talking about cool science!
These cool color changes are brought about by chemistry, specifically by a little number known as pH. While it is pretty common knowledge that this number indicates how acidic something is, understanding acidity is a little more complicated but surprisingly elegant! Let’s start by exploring a surprising fact about water! This precious little molecule known as dihydrogen monoxide has a secret: it doesn’t always stay as H2O! As these tiny water molecules collide, they sometimes exchange hydrogen atoms, like this:
Animation by Dr. Walt Volland
This is an equilibrium, meaning the reaction is constantly going both directions. The H30+ ions are called hydronium and cause water to be more acidic. The OH– ions are called hydroxide and make water more alkaline or basic.
CAUTION: MATH AHEAD!
In distilled water, which doesn’t contain anything else, this happens so that each of these has a concentration of 10-7, which is very very small. pH is just the negative power of the concentration of hydronium in this concentration, so distilled water has has a pH of 7, which is neutral. All of this is based on the equilibrium in the animation above.
Conveniently, the concentrations of the two always have their powers add up to -14. This means if you add a bunch of acid to some water to get a pH of 1 (which would be a concentration of 10-1), the concentration of the OH- ions would be 10-13. This should make some sense: If you pour a bunch of acids and bases together, you don’t end up with something that is an acid and a base. Instead, they combine to form water, until there is a little of one left over.
Unfortunately, just looking at a pool of water isn’t enough to tell if it is acidic or basic, since our eyes can’t see the tiny ions that are responsible floating around. Instead, we used something called Universal Indicator, which changes color depending on the pH of the solution it is in. Those colors can reveal what the solution is like by using the chart below.
The preliminary blue heart matches up with about a pH of 10. Since this is greater than 7, we know the solution is basic (or alkaline). Next, a spritz of vinegar is added, resulting in a change of heart. The color shifts to a pinkish color, which is on the acid side of the scale.
Universal indicator is not the only way to see pH. The second heart starts with a clear liquid, which actually contains an indicator called phenolphthalein. This indicator remains clear at all pH’s above 8.2, and between 10 and 13 it changes to a very holiday appropriate fuschia! To get this result, we simply added a dilute ammonia solution!
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